High frequency electronic signals need transmission lines to minimize distortion and attenuation. For transmission over long distances, a coaxial cable may be used. For transmission over short distances within circuit modules, there are three common transmission line configurations, all referred to by the generic term "strip-type transmission line." The first strip-type transmission line configuration is a strip conductor above an extended ground plane (also called a microstrip). The second configuration is a strip conductor between (coplanar) extended parallel conducting surfaces. The third configuration is a strip conductor embedded within a dielectric substrate with extended ground planes on the top and bottom surfaces of the substrate (also called a stripline). The present invention is primarily concerned with the first two strip-type transmission line configurations; the microstrip configuration and the coplanar configuration.
Ideally, for any multiple segment transmission-line system, each segment of the transmission line should have the same characteristic impedance. This is not always practical, however, in a real system. A transition from a microstrip or a coplanar transmission line to coaxial cable creates an unavoidable discontinuity in the electric fields which results in a discontinuity in the effective transmission line impedance and signal distortion due to reflections. This distortion may be minimized by proper transition geometry.
If the length of a transition path (connector) is much shorter than the distance a signal propagates during a time interval equal to the signal rise time, the transition path (connector) can be treated as a lumped element. If the length of a transition path is on the order of the distance a signal propagates during a time interval equal to the pulse rise time, the transition path must be treated as a transmission line. At gigahertz clock rates and picosecond rise times, the length and shape of a connector becomes important. A connector is needed which minimizes reflections by providing a non-abrupt transition from a coaxial cable to a microstrip or coplanar transmission line.